HR: 1340h
AN: MR23C-1521    [Abstracts]
TI: A Computational Study of Ionic Vacancies and Diffusion in MgSiO3 Perovskite and Post- perovskite
AU: Khanduja, G
AF: Department of Computer Science, Louisiana State University, 298 Coates Hall, Baton Rouge, LA 70803, United States
AU: * Karki, B B
EM: karki@csc.lsu.edu
AF: Department of Computer Science, Louisiana State University, 298 Coates Hall, Baton Rouge, LA 70803, United States
AB: We have performed first-principles simulations within density functional theory to investigate the effects of pressure on the formation of defects (ionic vacancies) and ionic diffusion in the perovskite (pv) and post- perovskite (ppv) phases of MgSiO3. Our results show that the predicted formation enthalpies of three Schottky (MgO, SiO2 and MgSiO3) defects are similar between the two phases at high pressures (100 to 150 GPa) with MgO Schottky defect being the most favorable. However, the calculated activation enthalpies and activation volumes of diffusion are shown to differ substantially between them. In particular, the activation enthalpies for Mg and Si diffusion in ppv are smaller than the corresponding values for pv, for example, by factors of 2.2 and 3.4, respectively, at 120 GPa, whereas the O migration enthalpy of ppv is only slightly larger than that of pv. The easy migration paths of the cations in ppv are shown to take place along the <100> direction in which Si-O octahedra share the edges. Visualization of the simulation data reveals that the vacancy defects and migrating ions induce substantial distortions in the atomic and electronic structures around them. It is suggested that diffusion is equally easy for all three species in ppv and is likely to occur through extrinsic processes near the bottom of the lower mantle. Our results are expected to be useful in modeling mantle conductivity and dynamics. Future work will focus on investigation of more complex mechanisms such as coupled diffusion or diffusion in the presence of other vacancies, and interstitial mechanisms. Our preliminary results show that in the presence of an O vacancy, the activation volume of Si diffusion becomes negative.
DE: 1025 Composition of the mantle
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3904 Defects
DE: 3924 High-pressure behavior
DE: 5120 Plasticity, diffusion, and creep
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting